This application is related to Japanese Patent Application No. HEI 11(1999)-265651 filed on Sep. 20, 1999, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave reflector, particularly to a surface acoustic wave reflector usable in a surface acoustic wave filter.
2. Description of Related Art
Surface acoustic wave filters have been used as filters in mobile communication terminals typified by portable telephones. With development of mobile communication systems, much higher quality is demanded of surface acoustic wave filters. A CDMA (code division multiple access) system, which has been increasingly introduced recently, requires IF (intermediate frequency) filters having extremely tough characteristic such as wider band, lower shape factor and better phase linearity than required by a conventional TDMA (time division multiple access) system.
To produce a small-sized filter which satisfies these strict demands, there is proposed a surface acoustic filter using a surface acoustic reflector having perturbation members arranged periodically (for example, metal electrode fingers of a grating structure) (see U.S. Pat. No. 5,379,010).
Such a surface acoustic wave filter is composed of a combination of a so-called interdigital transducer (referred to as IDT hereinafter) for input and output and a surface acoustic wave reflector (referred to simply as a reflector hereinafter). The frequency characteristic of the surface acoustic filter is a resultant of addition of the frequency characteristics of the IDT and the reflector.
For obtaining a desired frequency characteristic, it is necessary to control the frequency characteristic of the reflector. For this purpose, conventionally, the perturbation members are weighted by withdrawal (i.e., selective withdrawal of perturbation members), or if the perturbation members are made of metal strips, they are weighted by varying their widths one by one (see U.S. Pat. No. 5,270,606).
However, weighting simply by thinning has a limit because this provides only a binary weighting according to the presence or absence of perturbation members. In weighting by varying the widths of metal strips one by one, manufacturing variations in the widths of the metal strips result in a distortion from a desired frequency characteristic, and consequently the production yield drops.
To sum up, it is difficult to obtain desired filter characteristic by the conventional ways of weighting the reflector.
An object of the present invention is to provide a surface acoustic wave reflector so constructed that, in the frequency characteristic required of the surface acoustic wave filter, a first sidelobe can be suppressed.
The present invention provides a surface acoustic wave reflector comprising a piezoelectric substrate and a plurality of perturbation members arranged at a first period on the piezoelectric substrate, wherein a plurality of free-surface regions where no perturbation members exist are formed in an arrangement of the perturbation members.
With this construction, the first sidelobe in the frequency characteristic of the reflector can be suppressed more effectively.